There is an enormous base of existing DS1 facilities in the world's telecommunications infrastructure, particularly in North America. Similarly, there is a very large base of existing E1 facilities in the world's telecommunications infrastructure, particularly in Europe. These facilities consist of millions of miles of copper wire pairs, thousands of communications office wire frames, millions of DS1 and E1 connectors; each constructed specifically to carry one or more signals with the characteristics of the traditional electrical DS1 or E1 link.
Each of these separate infrastructures is designed to transport DS1 or E1 signals based on the known and understood electrical characteristics of the traditional, ternary, Alternate Mark Inverted (AMI) line coded 1.544 Megasymbols/sec DS1 format or the 2.048 Megasymbols/sec HDB3 format. The implemented binary and ternary data capacity of the AMI line coded DS1 electrical format is 1.536 Msymbols of payload bits plus 8 Ksymbols of DS1 framing bits. The implemented binary and ternary data capacity of the HDB3 line coded E1 electrical format is 1.920 Megasymbolss of payload bits plus 64 Ksymbols of E1 framing bits and 64 Ksymbols of CRC/Signaling.
Although an alternative, it is only marginally possible to increase the binary capacity of the existing DS1 or E1 infrastructures by simply increasing the symbol rate of the ternary encoded electrical signals above the existing symbol rates. The planning rules for the whole of the DS1 and E1 infrastructures are respectively based on the electrical characteristics of the ternary 1.544 Megasymbol/s DS1 and 2.048 Megasymbol/s E1 electrical signals, and the introduction of higher line rates would result in unacceptable changes in parameters such electrical crosstalk between wire pairs in inter-office and intra-office cable bundles or connection panels. The unacceptable changes in these parameters prevent increases in binary payload capacity within those infrastructures from being expanded by simply increasing the line symbol rate.
In the current art, the AMI line coding format for each electrical DS1 link codes 1.544 Megasymbols of DS1 binary coded data into 1.544 Megasymbols of ternary coded data. Similarly, the HDB3 line coding format for each electrical E1 link codes 2.048 Mbits of E1 binary coded data into 2.048 Msymbols of ternary coded data. The existing AMI and HDB3 line coding techniques assign binary payload values to only 2 of the potential 3 electrical ternary states. The 0 ternary symbol is equated to the 0 binary bit, but more relevantly, both the + and - ternary codes are assigned to the binary 1 symbol. There are minor exceptions to this simple assignment between binary and ternary symbol . Specifically, an exception is when ternary 0 symbol are changed to ternary + or - codes as a part of the HDB3 algorithm to limit the number of consecutive zeros appearing on the E1 line. These exceptions serve only to limit the number of consecutive ternary zeroes and do not increase the binary payload capacity of the E1 line. This assignment of one binary state to two ternary symbol has some benefits in the current art, in that by alternating the polarity of the + or - ternary symbols corresponding to successive "mark" bits (each binary 1 bit is referred to as a "mark"), the low frequency content of the electrical spectrum can be minimized, specifically reducing the DC energy on the line due to the transmitted datastream to a zero content. However, this AMI coding technique suffers from the inefficiency of basically throwing away one third of its potential capacity to transport binary data.
There exist in prior art other ternary line coding techniques which can satisfy the need for controlling the content of low frequency electrical energy while maintaining an efficient binary transport capacity. See P. A. Franaszek, "Sequence-State Coding for Digital Transmission", Bell System Technical Journal, December 1968, pp. 143-157. A relevant example of this is the 4B/3T (4 binary symbols/3 ternary bits) line coding technique discussed in more detail below. As mentioned above, for AMI coding, 4 ternary symbols are required to represent each 4 binary bits in a datastream. The 4B/3T coding technique requires only 3 ternary symbols to represent each 4 bits of binary data. Therefore the 4B/3T line coding technique can transport 33 percent more binary data per ternary symbol than AMI or HDB3. But no such prior art coding technique can be used to achieve a desired maximum increase in binary payload capacity while simultaneously maintaining the correct bandwidth for existing framing techniques for DS1 and E1 infrastructures.
The T148 product line of the then ITT Telecommunications, Inc. used 4B/3T coding over the whole of a 2.370 Mbit T148 electrical transmission link to realize a 3.088 Mbit binary payload rate over a 2.370 Megasymbol ternary rate transmission link (2.364 ternary Msymbols for 3.152 binary Mbits for T148C). See E. E. Schnegelberger and P. T. Griffiths, "48 PCM Channels on T1 Facilities", National Electronics Conference, 1975, pp. 201-205. Other line coding techniques, such as duo-binary and 4B2Q are used in various transmission lines. However, all of these examples suffer from the same shortcoming, the electrical characteristics at each point in the DS1 or E1 infrastructure, primarily the amplitude and power spectrum of crosstalk experienced between co-located copper pairs in multi-pair cable, prevent the application of these other line coding techniques to the bulk of the existing DS1 and E1 infrastructures. Therefore, these other potential line coding techniques are unable to be made use of in the existing base of DS1 and E1 infrastructures.